This was made to sit inside the main loop of the game/realtime-demo as that was what was blocking the repl from updating. Luckily we are in control of the loop so that is easy. Now you are wanting to use this in your library (probably with opencv) so you need to identify what is blocking the update, this is generally some kind of 'main loop' if you have access to the main loop then call update-swank from in there.
If the main loop is being controlled by a foreign library then perhaps you can put it in a function that gets called every 'loop'. All you really need is it to be called often enough that the repl feels responsive.
Other than that you can try changing the settings of swank to run on a separate thread though I have no experience doing that so I cant tell you how well that would work.
You can see it inside the main loop in the (run-demo) function of this example
Also I talk about it in this video. Though albeit perhaps not in enough detail :)
The fact you are getting "swank does not exist" is very odd. It suggest that swank is not loaded, but if you are using slime then swank must be there!
p.s. Remember that this is for use with Slime or Slim, which means you are using them with emacs or vim. If you are not using Slime+emacs or Slim+vim this function will not work!
[EDIT] Ok so I duplicated your issue by putting (ql:quickload :cepl) at the end of quicklisp's setup.lisp file. This gave me the 'Package SWANK does not exist'. One quick way to address this is to specify swank as a dependency in your project's asd file. So for example:
(asdf:defsystem #:cepl
:serial t
:depends-on (#:cl-opengl
#:swank ;;<---HERE
#:lbm-sdl
#:varjo
#:cl-utilities
#:cl-ppcre
#:symbol-munger
#:classimp
#:temporary-file
#:md5)
:components ((:file "package")
(:file "maths/base-maths")
(:file "base-macros")
;; (:file "base-lispbuilder")
(:file "cepl-utils")
Hope it helps :)